Ultrasound imaging system using beamforming techniques for phase coherence grating lobe suppression

ABSTRACT

High-frequency ultrasound imaging can be performed with greater quality and suppressed grating lobes by using methods and systems for effectively reducing the temporal length of transmit grating lobe signals in received ultrasound echoes. Systems and methods are provided for improved high-frequency ultrasound imaging. In various aspects, the method of shortening the time domain of grating lobe signals comprises splitting an array of N transmit elements into K sub-apertures. In further aspects, the grating lobes are suppressed by performing signal processing of the shortened grating lobe signals. In certain aspects, the signal processing method comprises weighting the samples by a calculated phase coherence factor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/302,242, filed Feb. 8, 2010, the entire disclosure of which is herebyincorporated by reference in its entirety for all purposes.

FIELD OF THE DISCLOSURE

This invention relates generally to ultrasound imaging systems, and moreparticularly to suppressing grating lobes in an ultrasound imagingsystem.

BACKGROUND

Low-frequency ultrasound imaging systems are very commonly used indiagnostic medicine, and they have been used for over 50 years. Newhigh-frequency ultrasound imaging technology offers dramaticimprovements in image resolution compared to these conventionallow-frequency systems. Notwithstanding the increased performance that ispossible with high-frequency ultrasound imaging, there are manytechnical barriers preventing its widespread use. Some of these barriersmay be addressed by using array-based systems for high-frequencyultrasound imaging, but fabricating transducer arrays and the associatedbeamformers is more difficult for high-frequency systems since muchsmaller dimensions are involved (e.g., the element to element pitch ofthe transducer).

If an array is fabricated without having sufficiently small dimensions,large image artifacts result called grating lobes. Another unsolvedproblem of existing systems is that there is no simple and effective wayto suppress grating lobes for ultrasound imaging systems that have arraytransducers with a large element-to-element pitch. One technique thathas been proposed for suppressing the grating lobes is described in J.Camacho, M. Parrilla, and C. Fritsch, “Phase Coherence Imaging,” IEEETrans. Ultrason., Ferroelectr., Freq. Control, Vol. 56, No. 5, pp.958-974, 2009. This technique, called “phased coherence imaging,”suppresses grating lobes using phase coherence correction factor receivebeamforming and synthetic aperture transmit beamforming.

Synthetic aperture beamforming is not suitable for use in high-frequencyultrasound imaging where small vibrations can create phase shifts in thereceived signals. Although synthetic aperture beamforming can producehigh frame rates for generating full 2D images, all of the elements needto be pulsed individually before the beamforming delays are inserted.This means that this beamforming technique is susceptible to imagedistortion due to the large amount of time expired during theacquisition of the pre-beamformed signals. This image distortion isavoided however when implementing transmit focal-zone beamforming.Although only one A-scan line can be collected per transmit event, imagedistortion due to small motion artifacts is avoided due to the smallamount of time expired between beamforming events. Unfortunately, forphase coherence imaging, transmit beamforming creates very long pulsesin the grating lobe region which, upon returning to the array elements,create very long narrow band receive pulses. Consequently, when phasecoherence correction factors are calculated from the received echoes inthe same temporal region as the main lobe, there are no longer anyrandom phases present since all of the long grating lobe echoes nowoverlap and for a certain time duration, are virtually all in-phase.

Thus, a need exists in the art for improved methods that effectivelyshorten the grating lobe signals in received ultrasound echoes, therebyenabling improved signal processing and suppression of grating lobes.

SUMMARY

The present disclosure addresses long-felt needs in the field ofultrasound imaging by providing systems and methods for effectivelyreducing the temporal length of transmit grating lobe signals inreceived ultrasound echoes. By shortening the grating lobe signals, thegrating lobes can subsequently be suppressed using signal processing,e.g., by application of a calculated phase coherence factor. In thisway, the present methods advantageously make possible the performance ofhigh-frequency ultrasound imaging with improved image resolution.

Various aspects of the present disclosure provide techniques fortransmit beamforming to be used with a phase coherence imaging techniquethat allow this technique to be used to suppress grating lobes in apractical, real ultrasound imaging system. The phase coherence imagingtechnique is enabled by using a transmit beamforming approach thateffectively shortens the time-domain signal of the received echoes. Insome aspects, the phase coherence imaging comprises sign coherencefactor (SCF) weighting. By producing shorter time-domain signals, thepresent methods create a situation in which a smaller number of thereceived echoes overlap upon being received by the imaging transducermaking the SCF weighting of the phase coherence imaging technique ismore effective.

Various techniques can be used to shorten this time-domain signal. Inone embodiment, the time-domain signal is shortened by splitting thetransmit signal using a newly developed “split aperture” technique. Inthe split aperture technique, the aperture is divided into a number ofsub-apertures, which are then selectively focused to obtain beamformedtransmit pulses that shorten the length of the time-stretched signal inthe grating lobe region. In another embodiment, the time-domain signalis shortened using a defocused “probing pulse” technique. Any suitabletechnique known in signal processing for shortening the time-domainsignal can be used to enable the use of transmit beamforming with thephase coherence imaging technique, which may be implemented in anultrasound imaging system.

Accordingly, various aspects of the present disclosure suppress gratinglobes in large pitch arrays without requiring synthetic aperturebeamforming. Using this technique for suppressing grating lobes, it ispossible to develop ultrasound imaging systems having array-basedtransducers with a larger pitch. The larger pitch may simplify thefabrication procedure of high-frequency transducers significantly, orreduce the number of required elements in 2D arrays resulting in arraysthat can beam-steer to lager angles with fewer elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) shows a schematic representation of conventional transducerarray having a linear array geometry. FIG. 1(B) shows a schematicrepresentation of conventional transducer array having a two-dimensionalarray geometry. FIG. 1(C) shows a schematic representation ofconventional transducer array having an annular array geometry.

FIG. 2 shows a schematic representation of beamforming using a lineararray.

FIG. 3 shows a schematic representation of beamforming using a phasedarray.

FIG. 4 shows a schematic representation of a geometrical arrangement ofan array and a desired focal point within an imaging medium.

FIG. 5 shows a schematic representation of receive beamforming.

FIGS. 6A-C show a comparison of grating lobe echoes for a 64 elementphased array with 1.25λ pitch using (FIG. 6A) transmit beamforming,(FIG. 6B) synthetic aperture pulsing with only the central element, and(FIG. 6C) split-aperture transmit beamforming K=2). The receivebeamforming delays have been inserted.

FIGS. 7A-B show proposed transmit and receive apertures according tovarious aspects of the disclosure. Each sub-aperture (K) is focusedseparately during transmission (FIG. 7A) and the echoes are received byall elements (FIG. 7B). One line of the image is constructed after allsub-apertures are pulsed.

FIG. 8 shows the geometry of a phased array transducer with aperturewidth (w), and element-to-element pitch (p) focused at focal point (F)on the main axis. The virtual curved aperture is used for calculatingthe distances between grating lobe point (G) and the aperture points(L1, L2, and L3) in order to account for the beamforming delays.

FIG. 9 shows a comparison of grating lobe levels for a 64-element phasedarray transducer with pitch (p)=1.25λ, focused at f/2, and steered at 25degrees between no SCF, SCF-weighted transmit beamforming, andSCF-weighted synthetic aperture.

FIG. 10 shows a comparison or grating lobe levels between split-aperturetransmit beamforming with different number of splits (K=1, 2, 4, 8). Thetotal aperture is a 64-element transducer with f=40 MHz, pitch(p)=1.25λ, f/2, steered at 25 degrees. As shown by increasing the K,grating lobe is suppressed more while PRF is decreased.

FIGS. 11A-C show the effect of K sub-apertures on grating lobesuppression for different element pitches and steering angles for a64-element phased array focused at f/2. The grating lobe level isplotted versus K “splits” (1, 2, 4, and 8) for steering angles 0, 15,30, and 45 degrees at element pitches of p=0.75λ (FIG. 11A), p=λ (FIG.11B), and p=1.25λ (FIG. 11C). The regular value of grating lobe with noprocessing (No SCF) is also plotted for comparison.

FIG. 12 shows an experimentally measured grating lobe transmit signalwhen the full 64-element aperture is active and focused off to 25degrees at f/2. The measurements were obtained using a 64-element 50 MHzphased array with 1.25λ element pitch.

FIG. 13 shows an experimentally measured grating lobe transmit signalwhen half of the aperture is active and focused off to 25 degrees atf/2. The measurements were obtained using a 64 element 50 MHz phasedarray with 1.25λ element pitch.

FIG. 14 shows experimentally measured radiation patterns from a 50micron wire phantom located at 25 degrees and f/2 when the beam is sweptfrom +35 degrees to −35 degrees. FIG. 14 shows (A) a radiation patternmeasured when no SFC is applied; (B) a radiation pattern when SCF isapplied; and (C) a radiation pattern when the aperture is split in two(K=2). The measurements were obtained using a 64 element 50 MHz phasedarray with 1.25λ element pitch.

FIGS. 15A-C show images generated with a 64 element 50 MHz phased arraywith 1.25λ spacing. The image is of a 50-micron wire phantom located atf/2.5. The image depth ranges from 1 mm to 8 mm and the steering angleranges from +/−35 degrees. All images are displayed with a dynamic rangeof 60 dB. FIG. 15A shows an image generated with no SCF processing; FIG.15B shows an image generated with SCF processing but no aperturesplitting on transmit; and FIG. 15C shows an image generated bysplitting the transmit aperture in two (K=2) and then applying SCFprocessing.

DETAILED DESCRIPTION

The present disclosure relates generally to systems and methods foreffectively reducing the temporal length of transmit grating lobesignals in received ultrasound echoes. The grating lobe signals can thenbe suppressed using a suitable signal processing method. These methodsand systems advantageously make possible the performance ofhigh-frequency and/or 2D ultrasound imaging arrays and providesignificant improvements in ultrasound image quality.

A small element-to-element pitch (˜0.5λ) is conventionally required forphased array ultrasound transducers in order to avoid large gratinglobes. This constraint can introduce many fabrication difficulties,particularly in the development of high-frequency phased arrays atoperating frequencies greater than 30 MHz. The present disclosureprovides a novel transmit beamforming technique that enables theperformance of high-frequency ultrasound imaging.

In various aspects, the present disclosure provides methods for highfrequency ultrasound imaging using a split transmit aperture, the methodcomprising the steps of: splitting a transmit beamformer comprising aphased array of N transmit elements into K sub-apertures, eachsub-aperture having N/K transmit elements; forming a focused ultrasoundtransmit beam from one of the sub-apertures of the transmit beamformer;transmitting the transmit signal towards a target along a focused lineof sight; obtaining samples of reflections of the transmit signal from atarget at all N elements of the full transmit aperture; and processingthe samples to produce an image of the target.

The present transmit beamforming technique can be used in conjunctionwith any suitable signal processing method, such as for example, phasecoherence imaging with sign coherence factor (SCF) receive beamforming(Camacho et al., IEEE Trans UFFC, 56(5):958-974 (2009)), which iscapable of suppressing grating lobes in large-pitch phased-arraytransducers.

In various aspects, methods are provided for splitting the transmitaperture (N elements) into N/K transmit elements and receive beamformingon all N elements to reduce the temporal length of transmit grating lobesignal. This method eliminates the need to use synthetic aperturebeamforming in phase coherence imaging. In certain aspects, the receivedsignals are weighted by the calculated SCF after each transmit-receiveevent to suppress the grating lobes. After pulsing all sub-apertures,the RF signals can then be added to generate one line of the image.Simulated 2-way radiation patterns for different K values have shownthat grating lobes can be suppressed significantly at different steeringangles. In some aspects, the present disclosure provides techniques fordetermining the optimal transmit sub-apertures has been developed.

Transducer Arrays

The structure of an array transducer is similar to that of singleelement transducers in many ways. For example, array transducers arecomposed of a piezoelectric sandwiched between a lossy backing layer anda matching layer(s). The piezoelectric resonator in an array transducer,however, is diced to produce a series of individual array elements.FIGS. 1(A), 1(B), and 1(C) illustrate the front faces of three commonarray geometries. The array shown in FIG. 1(A) is a linear array, thearray shown in FIG. 1(B) is a two-dimensional (2-D) array, and the arrayshown in FIG. 1(C) is an annular array.

Linear array transducers, such as the example shown in FIG. 1(A), havethe ability to focus the ultrasound energy at any depth in the tissue,along a line parallel to the row of array elements. The ability to focusultrasound energy at any depth in the tissue makes linear arraytransducers more attractive than single element transducers because thedepth of field is greatly increased. The ultrasound beam is passivelyfocused in the elevation direction (perpendicular to the row ofelements) using an acoustic lens or geometric curving. There are twotypes of linear arrays: one referred to as a “linear array” and theother referred to as a “phased array.”

“Linear arrays” focus the ultrasound beam perpendicular to the arrayusing a sub-aperture of array elements. FIG. 2 illustrates a group ofarray elements 201 used to form an active aperture. The group of arrayelements are excited using a pattern of delayed excitation signals 210to produce ultrasound wavefronts 220 that are focused along an imageline 222 perpendicular to the array. Additional image lines are obtainedby shifting the active aperture across the array. A sub-aperture ofelements steps across a much larger aperture, collecting the parallelA-scans needed to produce a 2-D image. A typical linear array will havea total aperture consisting of 256 elements, and use a sub-aperture of64 elements with wavelength spacing λ between the array elements 201.

FIG. 3 illustrates the second version of a linear array, the “phasedarray,” that has the ability to steer the ultrasound wavefronts 320. Theelements 301 in the phased array are excited using a pattern of delayedexcitation signals 310 that focuses and steers the ultrasound wavefronts320. Consequently, the image line 322 is no longer perpendicular to thearray. Additional image lines are obtained by changing the steeringangle. By steering the ultrasound beam at different angles, a series ofA-scans are collected. These A-scans are used to generate a sectorformat image. As a result, phased arrays can have a large field of viewwith a relatively small aperture. Typically, a phased array will use 128elements with half-wavelength spacing between the array elements 301.Generally, other than the smaller element spacing and aperture size,phased arrays are similar to linear arrays.

Although annular arrays, such as the example shown in FIG. 1(C), aresuitable for many topical applications in high-frequency imaging, due totheir relatively large element sizes and low element counts, they do nothave the ability to beam steer or translate the aperture electronicallyand therefore need to be mechanically scanned. This means that the fixedaperture needs to be relocated in space in order to generate theparallel “lines of sight” that make up a 2D image. This creates a larger“effective” aperture limiting the packaging size, image scan window, andframe rate. High-frequency linear phased array transducers can overcomemany of the problems inherent to annular arrays. For example, sincephased arrays require no aperture translation arrays that are 3 mm orless in total aperture can be manufactured.

Transmit Beamforming

It is convenient to separate an ultrasound beamformer into two parts:the transmit beamformer, which generates the sequence of high voltagepulses required to excite the array and focus the transmitted energy;and the receive beamformer, which focuses the received signals. Theoperation of the transmit beamformer will be described with reference toFIG. 4.

FIG. 4 illustrates a geometrical arrangement of an array of elements 1through n (in cross-section) that are each separated by a distance d,and a desired focal point, target 444, within an imaging medium. Thelines connecting the transducer array elements 1 through n to the target444 show the paths from each element to the target 444. In order tofocus the transducer radiation to a target 444, the path lengthdistances from each of the transducer elements 1 through n to the target444 must be determined. Then the delay pattern to apply to signals tothe transducer elements 1 through n that is required to focus the soundwaves to the target 444 can be determined.

The path length from each of the transducer elements 1 through n to thetarget is calculated based on geometric analysis.l _(n)(x,y)=√{square root over ((y−d _(n))² +x ²)}  Eqn. (1)In Equation (1), l_(n) is the distance from the nth transducer elementto the desired (x,y) coordinate. If a constant speed of sound within themedium is assumed, the total time it takes a pulse to travel from thenth transducer element to the target is l_(n)/c_(o), wherein c_(o) isthe assumed speed of sound within the medium.

In order to create constructive interference at the desired focaldistance, a delay pattern is inserted so that all the pulses fromtransducer elements 1 through n arrive at the target 444 at the sametime. These delays are calculated by subtracting the maximum element totarget flight time given by Equation (2).

$\begin{matrix}{{\Delta\;{\tau_{n}\left( {x,y} \right)}} = {\frac{\sqrt{\left( {y - d_{n}} \right)^{2} + x^{2}}}{c_{o}} - \frac{\sqrt{y^{2} + x^{2}}}{c_{o}}}} & {{Eqn}.\mspace{14mu}(2)}\end{matrix}$In Equation (2), Δτ_(n) corresponds to the excitation delay for elementn. Because a transmit beamformer can only focus at one depth for eachtransmit event, the transmitted wave is allowed to disperse beforesubsequent transmit pulses are applied.Receive Beamforming

Analogous to the transmit beamforming; the radiation pattern that isreceived by the array can also be focused. The echo from a small objectin the body will arrive back at different array elements at slightlydifferent times. By delaying the signals from different elements toaccount for the difference in arrival times, the echoes can bere-aligned so that they will add coherently. A flow diagram of receivebeamforming is shown schematically in FIG. 5. The transducers (arrayelements 501) receive the reflected wave 505 and the signals producedare delayed in a phased pattern using delay devices 550 to createconstructive interference upon summation at adder 555.

The receive beamforming process is similar to transmit focusing with adifference: in transmit focusing, pulses can only be focused to onedepth in the tissue at a time, whereas in receive beamforming it ispossible to dynamically change the delay pattern applied to the echoesas they are received. In a sense, receive beamforming allows one toapproximate the radiation pattern of a geometrically shaped transducerwhose geometric focus is sweeping forward at the speed of sound. Liketransmit beamforming, the delay pattern for the transducer elements inthe array 501 is related to the time of flight between the element andthe target.

Phased Array Transducers

Phased array transducers can provide a large field of view with a smallaperture. However, a small pitch (˜0.5λ) is conventionally required forphased array transducers in order to avoid large grating lobes. Thisproduces huge fabrication challenges for high frequency phased arrays.The present disclosure provides a novel method for ultrasound imaging inwhich splitting the transmit aperture into K sub-apertures generatesbroader band grating lobe echoes. By applying a suitable signalprocessing method, such as for example, the previously described SCFweighting coefficients, grating lobes can be significantly suppressedover a conventional transmit beamforming technique with large pitcharrays. Using basic geometric principles, an expression for the optimalaperture splitting location can be derived that will produce equallyshort transmit pulses in the grating lobe region for the differentsub-apertures. Splitting the aperture into equal-width sub-aperturesclosely approximates the optimal splitting locations for most f-numbersand grating lobe angles. According to the present disclosure, the use ofa larger number of sub-apertures (K) can increase the amount of gratinglobe suppression for different pitches and steering angles. Byincreasing the steering angle, greater values of K are required foracceptable grating lobe suppression. Therefore, the number of splitapertures (K) should be chosen based on the steering angle and desiredimage contrast (grating lobe level) for the individual application. Thepresent methods enable high-frequency phased array transducers to bedeveloped with larger element-to-element pitch, which simplifies devicefabrication significantly.

High-frequency ultrasound imaging (i.e., >20 MHz) can provide highresolution images of micro-scale tissue structures (Lockwood et al.,Ultrasound in Medicine and biology, 15(6):60-71 (1996)). The currentcommercially available systems are mostly limited to intravascular andsmall animal imaging applications. The relatively slow expansion intonew clinical applications of high-frequency ultrasound can mostly beattributed to the difficulties in developing array-based transducers andbeamformers operating at these frequencies. Conventionally,high-frequency ultrasound imaging systems have been based onsingle-element transducers, which introduce a trade-off between lateralresolution and depth-of-field. Mechanical aperture translation is alsoneeded in this case to capture a full 2D image. Recent effort hasfocused on the development of high-frequency annular and linear arraytransducers (Cannata et al., IEEE Trans UFFC, 53(1):224-236 (2006);Brown et al., IEEE Trans UFFC, 51(8):1010-1017 (2004); Brown et al.,IEEE Trans UFFC, 54(9):1888-1894 (2007); Lukacs et al., Proc IEEE UFFC,105-108 (2005); Ritter et al., IEEE Trans UFFC, 38(2):48-55 (2002);Ketterling et al., IEEE Trans UFFC, 52(4):672-681 (2005); Snook et al.,Proc IEEE Ultrason Symp, 1:865-868 (2003); Hu et al., Proc IEEE UltrasonSymp (2009); Sisman et al., Proc IEEE Ultrason Symp (2009)). Althoughhigh-frequency annular arrays have been shown to provide largedepth-of-field and high-quality images, they also require mechanicalspatial translation, which can limit the frame rate and packaging size.The development of high-frequency linear array transducers has proven toovercome limitations in frame-rate previously introduced by themechanical translation, however, the field-of-view and packaging size islimited to the size of the full aperture since linear arrays can onlyfocus the ultrasound beam perpendicular to the array and do not have theability to beam-steer. In order to overcome the tradeoff betweenfield-of-view and packaging size, the development of a high-frequencycurvilinear array has recently been reported (Hu et al., Proc IEEEUltrason Symp (2009)). Although arrays such as these are indeedpromising, a more efficient method of overcoming the tradeoff betweenfield of view and aperture size can be achieved with a phased arraytransducer.

Phased array transducers have the ability to beam-steer and do not needto electronically translate a sub-aperture in order to generate parallelA-scan lines. Unfortunately, developing high-frequency phased arraytransducers has proven to be extremely difficult due to the difficultiesin fabrication. Specifically, in order to steer the ultrasound beam, theelement-to-element pitch needs to be significantly reduced in order toavoid the introduction of grating lobes (Cobbold, Foundations ofbiomedical ultrasound, 437-450 (2007)). For example, at 50 MHz and asteering angle of 45 degrees, in order to push the grating lobe angle to90 degrees, the element pitch needs to be reduced to 15 microns (Ziomek,Fundamentals of acoustic field theory and space-time signal processing,528-532 (1955)), which is beyond most current fabrication capabilities.For this reason, many studies have investigated different methods forgrating lobe suppression to allow design of phased arrays with largerpitch (Rew et al., Electronics letter, 19(19):1729-1731 (1993); Gavrilovet al., IEEE Trans UFFC, 44(5):1010-1017 (1997); Wang et al., IEEE TransAntennas and Propagation, 56(6) (2008); Ustuner et al., U.S. Pat. No.7,207,942 B2 (2007); Li et al., IEEE Trans UFFC, 50(2):128-141 (2003)).

Grating Lobe Suppression

Any suitable signal processing method for suppressing grating lobes canbe used according to the present disclosure, including methods currentlydescribed in the literature for suppressing grating lobes in large-pitchphased array transducers. According to various aspects of the presentdisclosure, the processing method comprises weighting the samples by acalculated phase coherence factor, which can comprise SCF.

One suitable signal processing method focuses primarily on manipulationof the array structure by removing the periodic pattern of the elements(Rew et al., Electronics letter, 19(19):1729-1731 (1993); Gavrilov etal., IEEE Trans UFFC, 44(5):1010-1017 (1997); Wang et al., IEEE TransAntennas and Propagation, 56(6) (2008)). In these methods, some elementsare removed randomly until an under-sampled portion of the apertureremains, resulting in a “sparse array.” However, there is a reduction intransmit intensity of sparse arrays because of the low number ofelements, which results in a low signal-to-noise ratio (SNR). The othermajor drawback to sparse arrays is that the level of the side lobes willincrease because the average side lobe to main lobe power is equal to1/N (Cobbold, Foundations of biomedical ultrasound, 437-450 (2007)).

According to one aspect of the present disclosure, the signal processingmethod can comprise a method for suppressing grating lobes that focuseson processing the echoes received by each element to suppress gratinglobes. According to these methods, a weighting factor (between 0-1) iscalculated based on a specific characteristic of echoes such astime-shift (cross-correlation (Ustuner et al., U.S. Pat. No. 7,207,942B2 (2007))) or the receiving direction of the echoes (FFT (Li et al.,IEEE Trans UFFC, 50(2):128-141 (2003))). The echoes are multiplied bythe computed weighting factors and added to generate one line of theimage. Although these methods are promising, they have the inherentdrawback of high computational cost in calculating the weightingfactors, which makes them unsuitable for high frame-rate imaging.

According to one aspect of the present disclosure, the signal processingcan comprise a low-computational power method called “phase coherenceimaging” for grating lobe suppression in large pitch arrays (Camacho etal., IEEE Trans UFFC, 56(5):958-974 (2009)). In this method, the phaseof delayed echoes received by each element is detected and then aweighting factor is defined based on the standard deviation of thephases at each time point. At the focal point, all of the element echoeswill be in phase, so the standard deviation of their phases is close tozero, which results in a weighting factor close to one. For the gratinglobes, the phases of the echoes are not always perfectly in phase, sothe standard deviation of them in certain cases is greater than zero,resulting in a lower weighting factor. This method is mostly effectivefor synthetic aperture beamforming where the received grating lobeechoes are broadband. Essentially, after the transmit beamforming delaysare reconstructed along with the receive beamformed A-scans, time domainpoints that are similar to the main lobe are either zero or random inphase over a large number of the elements. This creates a spread in thestandard deviation of phases and therefore the broad bandwidth of thereceived echoes is the primary reason that the standard deviation of thephases is non-zero.

Shortening Transmit Grating Lobe Signals

The present disclosure provides novel methods for generalizing the phasecoherence imaging method for suppressing grating lobes of phased arraytransducers when using transmit beamforming, where long narrowbandgrating lobe echoes are inevitable. The present disclosure relatesgenerally to systems and methods for effectively reducing the temporallength of transmit grating lobe signal in received ultrasound echoes.The benefits of grating lobe suppression through signal processing aresignificantly improved by decreasing the time-domain signal of thegrating lobe signal prior to signal processing. Using these methods,phased arrays with element pitches much larger than one-half of theultrasound signal wavelength are possible. Therefore, the fabrication ofhigh-frequency phased arrays is significantly simplified, and the numberof elements required in 2D arrays is reduced.

A special case of phase coherence imaging is calculating sign coherencefactor (SCF) as the weighting factor. In this method, the sign bit ofreceived echoes by each element (b_(i)) at each time point is considered(Camacho et al., IEEE Trans UFFC, 56(5):958-974 (2009)). At each timepoint, the standard deviation of sign bits (σ) is calculated and the SCFis defined as follows in Equations (3A) and (3B):

$\begin{matrix}{{S\; C\; F^{\alpha}} = {{1 - \sigma}}^{\alpha}} & {{Eqn}.\mspace{14mu}\left( {3A} \right)} \\{\sigma = \sqrt{1 - \left\lbrack {\frac{1}{N}{\sum\limits_{i = 1}^{N}b_{i}}} \right\rbrack^{2}}} & {{Eqn}.\mspace{14mu}\left( {3B} \right)}\end{matrix}$Where α≧1 adjusts the sensitivity of the correction factor and N is thenumber of elements. Although it has been shown that different “α” valuescan actually further suppress grating lobes, for the remainder of thisarticle we assume an α value equal to 1. At the focal point, where allof the received echoes are in phase, the sign bits of all elements arethe same, resulting standard deviation close to zero and as aconsequence the weighting factor equal to one. When transmit beamformingis used, the signal in the grating lobe region in a one-way radiationpattern is very long in the temporal domain. Therefore, when consideringechoes received by the array elements from the grating lobe region, theyare also very long and narrow-band. Even after the beamforming delaysare inserted, these long grating lobe echoes overlap and if the sign bitis considered in the same temporal region as the main lobe signal, thesignals are all similar in phase and hence the weighting factor in thiscase is also approximately equal to 1.

By shortening the time-domain of the grating lobe signals, the presentmethod makes possible the performance of high-frequency ultrasoundimaging at greater image resolution. Using the present methods, it ispossible to perform the method at ultrasound frequencies greater than 30MHz. In some aspects, the method can be performed at frequencies of 20MHz, 30 MHz, 40 MHz, or 50 MHz. The present methods are also applicableto low frequency ultrasound, such as at frequencies below 20 MHz.

FIG. 6(A) shows an example of the individually received echoes from thegrating lobe region resulting from transmit beamforming. The signals arefor a 64 element phased array with an element pitch of 1.25λ steering atan angle of 25 degrees and focusing to f/2. The pulse echoes weresimulated using the two-way impulse response method (San Emeterio etal., J Acoust Soc AM, 92(2):651-662 (1992)). The sum of the one-waytransmit pulses in the grating lobe region is calculated and then usedas the point source for the received echoes. The bandwidth of thetwo-way pulse echo in the main lobe region however is approximately 50%.FIG. 6(A) clearly shows how the overlapping echoes from the grating loberegion are stretched out in the time domain. Because they are virtuallyall in phase over a temporal window similar to the main lobe, a largeweighting factor results. This prevents SCF from effectively suppressinggrating lobes when transmit beamforming is used.

The SCF method can, however, effectively suppress grating lobes whensynthetic aperture transmit beamforming is used. The main difference isthat for synthetic aperture, only one element is pulsed at a time whichresults in broadband echoes returning to the array, even from thegrating lobe region. After the receive beamforming delays are inserted,these broadband echoes have very small overlap in the time domainresulting a large sign-bit standard deviation since many of the signalsare zero (random phase) at any given moment in time. This produces avery low SCF weighting coefficient. FIG. 6(B) shows an example of thereceived grating lobe echoes from a 64 element, 1.25λ pitch phased arraysteering to 25 degrees when pulsed with a single defocused element(element 32). In this case, it can clearly be seen that the echoes arepredominately not in phase in the same temporal region as the main lobesignal. In fact, the received signals are so broad band that most arezero (random phase).

As addressed above, synthetic aperture beamforming has significantdisadvantages because many transmit events are required before thesignals are beamformed. Therefore, the pre-beamformed signals aresusceptible to phase distortions from small tissue movements during therelatively long pulsing sequence. High-frequency arrays are particularlysensitive to small tissue movements since the wavelengths are extremelyshort and therefore a small amount of tissue motion results in a largechange in the echo phase. Transmit beamforming avoids these phasedistortions because long pulsing sequences between beamforming are notrequired. The methods of the present disclosure advantageously shortenthe time-domain of grating lobes without requiring the use of syntheticaperture beamforming.

If transmit beamforming is desired for large pitch phased arrayshowever, a new method for increasing the effectiveness of the SCF isneeded. Since the underlying problem in applying SCF to an array usingtransmit beamforming is the long time-stretched signal resulting in thegrating lobe region, a method that produces shorter time-domain signalsshould result in a smaller number of the received echoes overlappingupon receive and hence the SCF weighting technique should be moreeffective. Since the length of the time domain signal is approximatelyequal to the difference in arrival times between the closest andfurthest elements in the array, we are proposing a very simple solutionof splitting the transmit aperture into K sub-apertures, where Kpotentially varies from 2 to N (number of elements) in order to shortenthe length of the time-stretched signal in the grating lobe region (FIG.7(A)). It is desirable to keep K as low as possible in order minimizethe total amount of time expired before the signals are beamformed.Again, this reduces the amount of phase aberration betweenpre-beamformed signals due to tissue motion.

According to the present disclosure, the number of sub-apertures (K) canbe any value such that the transmit aperture is capable of producing afocused beam. In certain aspects, K is an integer between 2 and 16. Infurther aspects, K is between 2 and 10. In yet further aspects, K is 2.

Any suitable element-to-element pitch can be used according to thepresent methods. According to certain aspects, the element-to-elementpitch is greater than 0.5λ. In further aspects, the element-to-elementpitch is 0.5λ. In certain aspects the element-to-element pitch is 0.75.In certain aspects the element-to-element pitch is 1λ. In certainaspects the element-to-element pitch is 1.25λ.

Any suitable steering angle can be used according to the presentmethods, depending on the value of the corresponding element-to-elementpitch. According to various aspects, the steering angle can be from 1 to45 degrees. In certain aspects, the steering angle is 10 degrees. Infurther aspects, the steering angle is 15 degrees. In further aspects,the steering angle is 20 degrees. In further aspects, the steering angleis 25 degrees. In further aspects, the steering angle is 35 degrees. Infurther aspects, the steering angle is 40 degrees. In further aspects,the steering angle is 45 degrees.

According the present disclosure, any suitable array size (N) can beused, for example and without limitation, the array size (N) can bebetween 16 and 512.

Unlike synthetic aperture beamforming which uses defocused pulses, themethods of the present disclosure use transmit focusing along differentlines of sight. In this case, N/K elements are pulsed with transmitfocusing delays and all N elements participate in the receive aperture(FIG. 7(B)). After each transmission, the SCF is calculated based on thetime-shifted echoes and is used to weight the beamformed signal. After Ktransmit events, all weighted echoes are added together to generate oneline in the image. Again, by reducing the size of the aperture to N/Kelements for transmission, the grating lobe signal is shorter due to thereduced difference in distance between the closest and furthest elementsin the transmit aperture. This reduction in overlap for the grating lobeechoes results in a much lower SCF. FIG. 6(C) shows an example of thereceived grating lobe echoes resulting from a split transmit aperture of32 elements after the receive beamforming delays have been inserted onall 64 receive elements K=2). Similar to FIGS. 6(A) and 1(B), thissimulation is for a phased array with an element pitch of 1.25λ, asteering angle of 25 degrees, and a focal depth of f/2. It can clearlybe seen from this plot that there is much less phase coherence betweenthe echoes for the split transmit aperture technique and will thereforeresult in a much lower SCF weighting factor.

According to various aspects of the present disclosure, the optimaltransmit apertures are determined in order to achieve equally shorttemporal transmit pulses between sub-apertures in the grating loberegion. Experimental simulation results have showed that approximatelyequal-width sub-apertures (½*w for K=2, ¼*w for K=4, etc.) produceapproximately equally short grating lobe transmit pulses or rather equaldifferences in distance between the closest and furthest element in thesub-aperture. An expression is derived below for determining where tosplit the transmit aperture in the case of K=2 in order to obtain equallength grating lobe signals from both transmit apertures. The optimalposition occurs when the difference in distance between the closest andfurthest elements in each sub-aperture are equal. FIG. 8 shows thegeometry of a phased array where “O” is the origin of the x-z Cartesianplane. It has been assumed that the focal point (F) is on the main axisand the grating lobe (G) is located on the same radius (R) at an anglefrom the central axis calculated by Equation (4) (t'Hoen, IEEE UltrasonSymp Proc, 94-95 (1982)).

$\begin{matrix}{\theta = {\sin^{- 1}\left( \frac{\lambda}{p} \right)}} & {{Eqn}.\mspace{14mu}(4)}\end{matrix}$In order to account for the effect of an array of elements with transmitbeamforming delays inserted in order to focus to F, a virtual curvedaperture is considered for the rest of the derivation. L1, L2, and L3are the distances between the grating lobe point (G) and the points onthe virtual curved aperture. In order to have transmit pulses with thesame length in the time domain for both splits, the equality ofdistances defined in (3) should be satisfied.

$\begin{matrix}{{{{L\; 3} - {L\; 2}} = {{L\; 2} - {L\; 1}}}{{L\; 2} = \frac{\left( {{L\; 1} + {L\; 3}} \right)}{2}}} & {{Eqn}.\mspace{14mu}(5)}\end{matrix}$where the distances between grating lobe and the virtual aperture pointsare:

$\begin{matrix}{{{L\; 1} = \sqrt{\left( {{R\;\sin\;\theta} - \frac{w}{2}} \right)^{2} + \left( {R\;\cos\;\theta} \right)^{2}}}{{L\; 3} = \sqrt{\left( {{R\;\sin\;\theta} - \left( {- \frac{w}{2}} \right)} \right)^{2} + \left( {R\;\cos\;\theta} \right)^{2}}}{{L\; 2} = \sqrt{\left( {{R\;\sin\;\theta} - x_{0}} \right)^{2} + \left( {{R\;\cos\;\theta} - z_{0}} \right)^{2}}}} & {{Eqn}.\mspace{14mu}(6)}\end{matrix}$where “w” is the total array aperture and (x₀,z₀) is point on thevirtual curved aperture. It can be shown that for L2, z₀ can be replacedas a function of x₀ reducing L2 to:

$\begin{matrix}{{L\; 2\left( x_{0} \right)} = \sqrt{\left( {{R\;\sin\;\theta} - x_{0}} \right)^{2} + \left( {{R\;\cos\;\theta} - \left( {R - \sqrt{\left. {R^{2} + \left( \frac{w}{2} \right)^{2} - x_{0}^{2}} \right)}} \right)^{2}} \right.}} & {{Eqn}.\mspace{14mu}(7)}\end{matrix}$and L1 and L3 are simply

$L\; 2\left( \frac{w}{2} \right)\mspace{14mu}{and}\mspace{14mu} L\; 2\left( {- \frac{w}{2}} \right)$respectively. The following derivation is based on the assumption that

$R\operatorname{>>}\frac{w}{2}$which is a reasonable assumption at f-numbers greater than 2. Bysquaring the right side of Equation (5), we obtain:

$\begin{matrix}{\frac{\begin{matrix}\left( {\sqrt{\left( {{R\;\sin\;\theta} - \frac{w}{2}} \right)^{2} + \left( {R\;\cos\;\theta} \right)^{2}} +} \right. \\\left. \sqrt{\left( {{R\;\sin\;\theta} - \left( {- \frac{w}{2}} \right)} \right)^{2} + \left( {R\;\cos\;\theta} \right)^{2}} \right)^{2}\end{matrix}}{(4)} \approx {R^{2} + \frac{R^{2}\cos^{2}\theta\frac{w^{2}}{4}}{\left( {R^{2} - \frac{w}{4}} \right)}}} & {{Eqn}.\mspace{14mu}(8)}\end{matrix}$The approximation is based on the first-order Taylor approximation of asquare:

$\begin{matrix}{\sqrt{x^{2} + a} = {x + \frac{a}{2\; x}}} & {{Eqn}.\mspace{14mu}(9)}\end{matrix}$By squaring the right side of Equation (5), and again using the Taylorapproximation in Equation (9), we obtain the expression:

$\begin{matrix}{\left( {{R\;\sin\;\theta} - x_{0}} \right)^{2} + \left( {{{R\;\cos\;\theta} - \left( {R - \sqrt{\left. {R^{2} + \left( \frac{w}{2} \right)^{2} - x_{0}^{2}} \right)}} \right)^{2}} \approx {{\left( {1 - {\cos\;\theta}} \right)x_{0}^{2}} - {\left( {2\; R\;\sin\;\theta} \right)x_{0}} + \left( {{\cos\;\theta\frac{w^{2}}{4}} + R^{2}} \right)}} \right.} & {{Eqn}.\mspace{14mu}(10)}\end{matrix}$The equality of Equation (8) and Equation (10) therefore results in

$\begin{matrix}{{{\left( {1 - {\cos\;\theta}} \right)x_{0}^{2}} - {\left( {2\; R\;\sin\;\theta} \right)x_{0}} + \left( {{\cos\;\theta\frac{w^{2}}{4}} - \frac{R^{2}\cos^{2}\theta\frac{w^{2}}{4}}{\left( {R^{2} - \frac{w^{2}}{4}} \right)}} \right)} = 0} & {{Eqn}.\mspace{14mu}(11)}\end{matrix}$By solving the root of Equation (11) and substituting R=F·w and w=N·p,the expression for “x₀” is obtained, which is the element at which tosplit the aperture in order to obtain equal-length time-domain signals.

$\begin{matrix}{x_{0} = {\left( \frac{{F\;\sin\;\theta} - \sqrt{{F^{2}\sin^{2}\theta} - {\left( {1 - {\cos\;\theta}} \right)\left( {\frac{\cos\;\theta}{4} - \frac{F^{2}\cos^{2}\theta}{\left( {{4\; F^{2}} - 1} \right)}} \right)}}}{\left( {1 - {\cos\;\theta}} \right)} \right)\left( {N\; p} \right)}} & {{Eqn}.\mspace{14mu}(12)}\end{matrix}$The term x₀ is a function of (N, F, p). Generally, however theexpression for x₀ approaches zero at very large and very small gratinglobe angles (i.e., the aperture is split at the central element).Intuitively, one can visualize a pulse arriving at the virtual curvedaperture either from 90 degrees or from the main axis. These pulseechoes will “see” a symmetric aperture where the difference between theclosest and furthest elements in each sub-aperture (split at x_(o)=0)are the same. In fact for most grating lobe angles, f-numbers, andelement pitches, x_(o) is typically very close to zero when K=2. In thismanner, the transmit beamforming technique is simplified making it easyto implement into a real time system. Similar expressions can easily beobtained for splitting the aperture into 3, 4, 5, or any other suitablevalue based on these simple geometric principles. Generally, however,splitting the aperture into equally sized sub-apertures closelyapproximates the calculated value.

Using the “equal aperture split” generalization, the “x” location thatshould be chosen for spit-aperture transmit beamforming is:

$\begin{matrix}\left\{ \begin{matrix}{{x_{i} = {{\left( {i + 1} \right)\frac{w}{K}} - \frac{w}{2}}},} & {0 \leq i < \frac{K - 2}{2}} \\{x_{i} = {- x_{{({K - 2})} - i}}} & \; \\{x_{\frac{({K - 2})}{2}} = 0} & {{if}\mspace{14mu}\left( {K - 1} \right)\mspace{14mu}{is}\mspace{14mu}{odd}}\end{matrix} \right. & {{Eqn}.\mspace{14mu}(13)}\end{matrix}$where, i is element number, K is the number of splits, w is the width ofaperture, and x_(i) is the coordinate of element based on the geometryin FIG. 8. It should be noted however that the derivation of Equation(12) was based on an approximation that is valid for f-numbers greaterthan approximately 2. In various aspects, f-numbers greater than 1 aresuitable for use with the present methods. In certain aspects, a focaldept of f/2 can be used.

By shortening the time domain of the grating lobe signals as describedabove, suppression of the grating lobes is dramatically improved usingsignal processing methods. In various aspects, the grating lobe signalcan be suppressed by between 20 dB and 60 dB. In certain aspects, thegrating lobe signal can be suppressed by 20 dB. In certain aspects, thegrating lobe signal can be suppressed by 40 dB. In certain aspects, thegrating lobe signal can be suppressed by 60 dB.

Ultrasound Imaging System

In various aspects, the present disclosure provides a system forhigh-frequency ultrasound imaging using a split transmit aperture, thesystem comprising: an imaging array comprising a phased array of Ntransmit elements, the transmit elements divisible into K sub-apertures,each sub-aperture having N/K transmit elements; a transmit beamformercoupled to the imaging array, wherein the transmit beamformer isconfigured to apply energy selectively to the elements of each of thesub-apertures to focus a transmit signal from the sub-aperture towards atarget; a receive beamformer coupled to the imaging array, wherein thereceive beamformer is configured to sample a signal received by theimaging array at each of the N elements thereof; and processingcircuitry configured to receive the sampled signal and compute an imagedbased thereon.

In various aspects, a computer controls the transmit beamformer. Thetransmit signals can comprise pulsed signals. The transmitted signalsreflect off of tissue structures (or target areas) and are received bythe elements in the imaging array. These signals received at the imagingarray can be directed through amplifiers that are connected between theelements of the imaging array. The digital data is transferred back tothe computer for image processing.

According to the present disclosure, the number of sub-apertures (K) inthe system can be any value such that the transmit aperture is capableof producing a focused beam. In certain aspects, K is an integer between2 and 16. In further aspects, K is between 2 and 10. In yet furtheraspects, K is 2.

Any suitable element-to-element pitch can be used according to thepresent systems. According to certain aspects, the element-to-elementpitch is greater than 0.5λ. In further aspects, the element-to-elementpitch is 0.5λ. In certain aspects the element-to-element pitch is 0.75λ.In certain aspects the element-to-element pitch is 1λ. In certainaspects the element-to-element pitch is 1.25λ.

Any suitable steering angle can be used according to the present system,depending on the value of the corresponding element-to-element pitch.According to various aspects, the steering angle can be from 1 to 45degrees. In certain aspects, the steering angle is 10 degrees. Infurther aspects, the steering angle is 15 degrees. In further aspects,the steering angle is 20 degrees. In further aspects, the steering angleis 25 degrees. In further aspects, the steering angle is 35 degrees. Infurther aspects, the steering angle is 40 degrees. In further aspects,the steering angle is 45 degrees.

According the present disclosure, any suitable array size (N) can beused, for example and without limitation, the array size (N) can bebetween 16 and 512.

Applications

The presently described methods can be used for any suitableapplication, such as for example endoscopy, which includes withoutlimitation, laproscopic, itra-cardiac, and surgical guidance imaging,and the like. Thus, the high-frequency ultrasound imaging systemdescribed herein can improve diagnostics, interventions, and therapeuticmonitoring of a variety of disorders. This new diagnostic imagingapproach can improve the objectivity and quality of diagnosis in thisfield of medicine, allowing physicians to apply more precisely targetedinterventions.

EXEMPLARY ASPECTS Example 1

SCF in Combination with Transmit Beamforming and Synthetic Aperture

The usefulness of SCF method for grating lobe suppression, is dependenton the temporal length of the transmit pulse in the grating lobe region.The shorter the transmit pulse, the more effective the SCF method is forgrating lobe suppression. In FIG. 9, 2-way radiation patterns for a64-element phased array transducer with element pitch (p)=1.25λ, focusedto f/2, steered at 25 degrees are shown. One radiation pattern has noSCF weighting and this is compared with SCF-weighted transmitbeamforming K=1) and SCF-weighted synthetic aperture beamforming. It canclearly been seen that SCF weighting is not very effective forsuppressing grating lobes when transmit beamforming is used, however, itsuppresses the grating lobes in synthetic aperture beamforming more than50 dB. Again the underlying reason for the big difference ineffectiveness between the two transmit techniques is seen in FIGS. 6(A)and 6(B). In FIG. 6(A), the grating lobe echoes in transmit beamformingare all in the same phase whereas they are not for synthetic aperture(FIG. 6(B)). For transmit beamforming, a weighting factor nearly equalto one results in the grating lobe region since all of the sign bits arethe same at all time points.

By splitting the aperture into two equal sub-apertures K=2), the signalsarriving from the grating lobe region are much shorter and as a result,the received echoes are not completely phase coherent after the receivebeamforming delays are inserted. FIG. 6(C) shows the received echoes onall 64 elements and it can clearly be seen that the sign bits are notall similar for the received signals and therefore the SCF weightingfactor is low. Similar to the received signals in synthetic aperturebeamforming, many of the signals are zero or random in phase at anygiven time point. By splitting the transmit aperture into more equalwidth sub-apertures, the length of the grating lobe signals becomes evenshorter resulting in an even lower SCF weighting factor.

Example 2

Effect of Aperture Splitting

FIG. 10 shows 2-way radiation patterns for a 64-element phased arraytransducer with pitch (p)=1.25λ, focused at f/2, and steering angle of25 degrees. Radiation patterns are compared between transmit apertureswith no weighting (No SCF), with SCF-weighting and no splitting (K=1),and SCF-weighting+splitting (K=2, 4, 8). This simulation clearly showsthat split-transmit apertures are very effective in grating lobesuppression with SCF weighting factors (e.g., 20 dB grating lobesuppression is achieved with K equal to only 2). By increasing K,smaller apertures are pulsed during transmission resulting in shortergrating lobe echoes and as a result have less phase coherence. However,the frame-rate is decreased by increasing K and more transmit events arerequired before the signals are beamformed, potentially resulting inphase aberrations.

For a more quantitative evaluation of the effectiveness of thesplit-aperture method on grating lobe suppression, 2-way radiationpatterns of 64-element transducers with different pitches (0.75λ, λ,1.25λ) steered at various angles (0, 15, 30, and 45 degrees) focused atf/2 are processed by SCF-weighting and different split-transmitapertures (K=1, 2, 4, 8). For each pitch value and steering angle, thegrating lobe level is plotted versus split-aperture (K) in order toobserve the effect of increasing K on grating lobe suppression. Fourdifferent steering angles for each pitch value are shown on each graphin FIG. 11, summarizing the results for p=0.75λ, λ, and 1.25λrespectively. At each angle, the grating lobe suppression increases byincreasing the number of split-apertures (K). We can see from FIG. 11that at large steering angles (30, 45 degrees) the amount of gratinglobe suppression increases by approximately 20 dB for all elementpitches (0.75λ, 1λ, and 1.25λ) by simply splitting the transmit aperturein half (K=2).

The important aspect of these graphs is that the K value should bechosen based on the range of steering angles in a given application andtransducer pitch. For example in the case of p=1λ, FIG. 11C shows thatwith K=1 (SCF-weighting with no splitting), it is possible to suppressthe grating lobe to less than −60 dB below the main lobe at a 15 degreesteering angle, while for a 45 degrees steering angle, K must beincreased to 8 in order to suppress the grating lobes to −60 dB.

As described above, increasing the K decreases the frame rate, which isusually undesirable. Therefore, a split aperture technique that could beused to recoup some of the decreased frame rate would be to graduallyincrease the “K” value as the A-scan lines shift to larger steeringangles. As shown in FIG. 11C even with an element pitch of 1.25, SCFweighting will suppress the grating lobe level approximately 60 dB belowthe main lobe at a zero degree steering angle with K=1 (no aperturesplitting). However, by the time 45 degrees of steering is reached, 8sub-apertures with transmit focusing are required to maintain the sameamount grating lobe suppression.

An alternative technique that could potentially avoid the need to usemultiple transmit pulses per A-scan line is to send out a broaddefocused “probing pulse” from the entire aperture in order to generatea map of SCF values for all space. In order to defocus the pulse,beamforming delays corresponding to a virtual point source behind thearray is required (Lockwood et al., IEEE Trans UFFC, 45(4):980-988(1998)). The echoes that are received from all points in space are nowvery broad band (short) and after receive beamforming delays areinserted along different A-scan lines, echoes from the grating loberegions will have low phase coherence and corresponding SCF weightingfactors. Since a broad defocused pulse is used upon transmission,dynamic receive focusing can be performed everywhere and hence a map ofSCF weighting factors could potentially be computed and stored in memoryfor all space from a single probing pulse. Then, if conventionaltransmit beamforming is carried out (one A-Scan line at a time), thesignals can be weighted with the previously computed weightingcoefficients from the initial defocused probing pulse. This technique ispossible since the weighting coefficients are slowly varying overdifferent regions in space and therefore are not overly susceptible tosmall amounts of tissue motion during the relatively long pulsingsequence.

FIGS. 12 and 13 show experimentally measured grating lobe transmitsignals when the full 64 element aperture is active and focused off to25 degrees at f/2 (FIG. 12) and when half of the aperture is active andfocused off to 25 degrees at f/2 (FIG. 13). The measurements wereobtained using a 64 element 50 MHz phased array with 1.25λ elementpitch. A comparison of FIGS. 12 and 13 demonstrates that the length ofthe grating lobe signal is reduced significantly when only half theaperture is used for transmit.

FIG. 14 shows experimentally measured radiation patterns from a 50micron wire phantom located at 25 degrees and f/2 when the beam is sweptfrom +35 degrees to −35 degrees. FIG. 14 shows (A) a radiation patternmeasured when no SFC is applied; (B) a radiation pattern when SCF isapplied; and (C) a radiation pattern when the aperture is split in two(K=2). The measurements were obtained using a 64 element 50 MHz phasedarray with 1.25λ element pitch. As seen in FIG. 14C, split-transmitapertures are effective in grating lobe suppression with SCF weightingfactors. Specifically, the level of grating lobes in this case aresuppressed more than 20 dB when the aperture is split in two. Thegrating lobe levels could be suppressed even further upon more aperturesplits.

FIGS. 15A-C show shows images generated with a 64 element 50 MHz phasedarray with 1.25λ spacing. The image is of a 50-micron wire phantomlocated at f/2.5. The image depth ranges from 1 mm to 8 mm and thesteering angle ranges from +35 degrees to −35 degrees. All images aredisplayed with a dynamic range of 60 dB. FIG. 15A shows an imagegenerated with no SCF processing; FIG. 15B shows an image generated withSCF processing but no aperture splitting on transmit; and FIG. 15B showsan image generated by splitting the transmit aperture in two (K=2) andthen applying SCF processing. As demonstrated by FIG. 15C, the use of asplit transmit aperture dramatically improves image quality, and whenused in conjunction with a processing method such as SCF, results insignificant suppression of grating lobes.

Although embodiments of the invention have been described with referenceto two-dimensional ultrasound imaging systems, these techniques may beapplied in other types of ultrasound imaging systems. For example, inview of this disclosure, one of skill in the art can employ thebeamforming and grating lobe suppression techniques in athree-dimensional ultrasound imaging system, without departure from theinventive concepts disclosed herein.

The foregoing description of the embodiments of the invention has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of theinvention in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by electrical circuits or equivalent computer programs,microcode, or the like, or any combinations thereof. The describedoperations and their associated modules may thus be embodied insoftware, firmware, hardware, or any combinations thereof.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the invention be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsof the invention is intended to be illustrative, but not limiting, ofthe scope of the invention, which is set forth in the following claims.

What is claimed is:
 1. A method of performing ultrasound imaging, comprising: transmitting a defocused pulse from an array of ultrasound elements, the defocused pulse having a virtual point source behind the array of ultrasound elements; receiving broadband reflections with the array of ultrasound elements, and obtaining signals from the reflections; applying receive beamforming delays to the signals and calculating phase coherence weighting factors along different A-scan lines, thereby obtaining a spatial map of phase coherence weighting factors; transmitting beamformed pulses from the array of ultrasound elements along one or more A-scan lines; and applying the spatial map of phase coherence weighting factors when performing receive beamforming on the signals to reduce grating lobes in an ultrasound image.
 2. The method according to claim 1 wherein the phase coherence weighing factors are sign coherence factors.
 3. The method according to claim 1 wherein the array of ultrasound elements has an element-to-element pitch greater than 0.5*lambda, wherein lambda is the wavelength of the beamformed pulses.
 4. An ultrasound imaging system comprising: an ultrasound imaging array comprising an array of ultrasound elements; a transmit beamformer operatively coupled to the ultrasound imaging array, wherein said transmit beamformer is configured to apply energy selectively to the ultrasound elements; a receive beamformer operatively coupled to the ultrasound imaging array, wherein said receive beamformer is configured to sample a signal received by said ultrasound imaging array at each of the ultrasound elements; and processing circuitry operatively connected to said transmit beamformer and said receive beamformer, wherein said processing circuitry is configured to: control said transmit beamformer such that a defocused pulse is transmitted from said ultrasound imaging array, the pulse having a virtual point source behind said ultrasound imaging array; receive, from said receive beamformer, signals produced from broadband reflections; apply receive beamforming delays to the signals and calculating phase coherence weighting factors along different A-scan lines, thereby obtaining a spatial map of phase coherence weighting factors; transmit beamformed pulses from the ultrasound imaging array along one or more A-scan lines; and apply the spatial map of phase coherence weighting factors when performing receive beamforming on the signals to reduce grating lobes in an ultrasound image.
 5. The system according to claim 4 wherein the phase coherence weighing factors are sign coherence factors.
 6. The system according to claim 4 wherein said ultrasound imaging array has an element-to-element pitch greater than 0.5*lambda, wherein lambda is the wavelength of the beamformed pulses. 